Creep high-temperature alloys

Creep curves -high ternperamre alloys [HIGH TEMPERATURE ALLOYS] (Vol 13)... 

Two important conclusions are reached from the Monkman-Grant equation. First, the most important engineering requirement to improve the lifetime of materials that fail by creep rupture is to improve creep behavior. Over the years, this has been the goal of much of the research on high temperature alloys. Second, theoretical treatments of creep rupture must be consistent with the Monkman-Grant equation, as has been shown by Ashby and Dyson.111... 

Vi >- B D S 1 p i 1 p H Ss a Timetal 6-2-4-2 Ti-6Al-2Sn-4Zr- 2Mo-0.08Si Good tensile creep and fatigue properties up to 540°C. It is the most commonly used high-temperature alloy in jet-engine compressors and airframe structures. 

Strain-rate sensitivity and flow stress results at 990 °C (1815 °F) for a near-a titanium alloy IMI 834 Cn-6Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.33Si) are shown in Fig. 13 for longitudinal and transverse orientations. IMI 834 is a high-temperature alloy with good tensile strength and creep resistance up to 600 °C (1110 °F). Compared to similar results for Ti-6A1-4V (Fig. 4), IMI... 

Lead—copper alloys are specified because of superior mechanical properties, creep resistance, corrosion resistance, and high temperature stabiUty compared to pure lead. The mechanical properties of lead—copper alloys are compared to pure lead, and to lead—antimony and lead—calcium alloys in Tables 4 and 5. 

Other alloys have been developed for use in particular corrosive environments at high temperatures. Several of these are age-hardenable alloys which contain additions of aluminum and titanium. Eor example, INCONEL alloys 718 and X-750 [11145-80-5] (UNS N07750) have higher strength and better creep and stress mpture properties than alloy 600 and maintain the same good corrosion and oxidation resistance. AHoy 718 exhibits excellent stress mpture properties up to 705°C as well as good oxidation resistance up to 980°C and is widely used in gas turbines and other aerospace appHcations, and for pumps, nuclear reactor parts, and tooling. 

Vessels for high-temperature serviee may be beyond the temperature hmits of the stress tables in the ASME Codes. Sec tion TII, Division 1, makes provision for construction of pressure vessels up to 650°C (1200°F) for carbon and low-alloy steel and up to 815°C (1500°F) for stainless steels (300 series). If a vessel is required for temperatures above these values and above 103 kPa (15 Ibf/in"), it would be necessaiy, in a code state, to get permission from the state authorities to build it as a special project. Above 815°C (1500°F), even the 300 series stainless steels are weak, and creep rates increase rapidly. If the metal which resists the pressure operates at these temperatures, the vessel pressure and size will be limited. The vessel must also be expendable because its life will be short. Long exposure to high temperature may cause the metal to deteriorate and become brittle. Sometimes, however, economics favor this type of operation. 

Metals Successful applications of metals in high-temperature process service depend on an appreciation of certain engineering factors. The important alloys for service up to I,I00°C (2,000°F) are shown in Table 28-35. Among the most important properties are creep, rupture, and short-time strengths (see Figs. 28-23 and 28-24). Creep relates initially applied stress to rate of plastic flow. Stress... 

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